1. Field of the Invention
The present invention relates to a method for fault diagnosis of a RF receive path in a cellular base station system, and more particularly, this invention relates to a method for diagnosing and localizing a fault in a receive unit of a digital cellular base station system.
2. Description of the Related Art
Both PCS and CDMA systems include a plurality of Base station Transceiver Subsystems (BTS) which offer a service to mobile stations, a Base Station Controller (BSC), a Base Station Manager system (BSM), a Mobile Switching Center (MSC) and a Location Registering System (LRS).
An area is called a cell where each base station offers a service and the cell is divided into several sectors. The coverage of the cell is expanded into a BTS area, a BSC area and a MSC area in order. A mobile station within each cell forms a channel with a BTS, which offers a service to the cell, performing communication. It is a forward channel that is formed from a base station toward a mobile station and a reverse channel that is formed from a mobile station toward a base station. Mobile stations transmit and receive voice information and data with a base station through a traffic channel.
In a mobile communication system, the subsystems under a mobile switching center are called base station subsystems (hereafter abbreviated as BSS). The BSS includes a base station manager system, base station controllers and base station transceiver subsystems in order of hierarchy. The main processor of the base station controller system is called a call control processor (hereafter abbreviated as CCP) and the main processor of the base station system is a BTS control processor (hereafter abbreviated as BCP).
A RF receive path in a CDMA digital cellular base station system refers to a path which receives a radio signal transmitted over a radio frequency (RF) from a mobile station. FIG. 1 is a block diagram illustrating a RF receive path in a typical CDMA digital cellular base station. As illustrated, a base station system 300 which offers service to mobile stations 200 includes an out-door system 90 having an antenna 10 which is coupled to the base station system via coaxial cable. The base station system 300 also includes an in-door system 80 which has RF transmitting and receiving equipment installed in the inner part of the base station.
The in-door system 80 includes a RF receive unit 50, a digital unit 60 and a base station test unit 70. The RF receive unit 50 generally includes a receive front-end stage 20, a transceiver unit stage 30 and an IF amplify & divide unit stage 40. The receive front-end stage 20 generally includes a receive front coupler, a band pass filter, a low noise amplifier and a power divider. Referring to FIG. 2, the transceiver unit 30 includes a receive down-converter board 410, a transceiver slave control board 420 and a transmit up-converter board 430. The digital unit 60 is generally formed with a digital signalling part.
The base station test unit 70 includes a RF switch unit (RSWU), an attenuator unit (ATTU) and a test mobile station unit (TSMU). The transceiver unit 30 can measure a level of a signal received through the antenna in the base station or the directional coupler of the receive front-end stage, directly connected to the antenna. FIG. 2 is a block diagram of a transceiver unit 30 in a receive path of a base station system. As illustrated, the transceiver unit 400 includes a receive down-converter board 410, a transmit up-converter board 430 and a transceiver slave control board 420. The receive down-converter board 410 includes a circuit for converting a received RF signal via a receive front-end stage into a receive intermediate frequency (IF) signal. The receive down-converter board 410 also includes an automatic gain controller (AGC) for maintaining a substantially constant receive IF level despite changes of the received RF input level.
The transceiver slave control board 420 monitors and controls circuit boards in the transceiver unit 400 such as the receive down-converter board 410, the transmit up-converter board 430, and itself. The transceiver slave control board 420 includes a microprocessor and a peripheral circuit. The transceiver slave control board 420 carries out functions such as setting a channel frequency of the corresponding transceiver unit 400, controlling an attenuator of a forward and a reverse link, storing the structure of a receive down-converter board and a transmit up-converter board, storing the fixed data about fluctuation of the variable attenuator, measuring a RF receive input signal intensity and monitoring a status of the transceiver unit 400.
The transmit up-converter board 430 includes a mixer circuit, a variable attenuator, a band pass filter and a local oscillator circuit. The mixer circuit converts a transmit IF signal transmitted from the digital signaling processor into a RF signal corresponding to a desired carrier frequency, in a manner well known in the art of RF circuit design.
In a CDMA digital cellular base station system, it is desirable to diagnose faults in an RF receive path in a base station during operation of the system. Such diagnostics are used to promptly detect faults and maintain system integrity. Therefore, a special fault diagnostic algorithm is needed for diagnosing the RF receive path.
Systems known in the prior art diagnose a fault in an outdoor base station system containing an antenna and a coaxial cable by measuring an antenna Voltage Standing Wave Ratio (VSWR). However, such systems are not generally applied to in-door base station systems and do not isolate faults within the indoor base station receiver.
U.S. Pat. No. 5,640,401 discloses a communication circuit fault detector. The disclosed fault detector temporarily stores a test signal generated from a sequence generator in a memory circuit and simultaneously inputs the test signal into the communication circuit. The fault detection circuit compares a returned test signal from the communication circuit with the test signal stored in the memory circuit. Unlike loop tests of the prior art, the invention disclosed in U.S. Pat. No. 5,640,401, tests a communication circuit during normal data communication. However, U.S. Pat. No. 5,640,401 is directed to fault detection of a general data communication circuit and does not teach a method for fault diagnosis of a receive path in a CDMA digital cellular base station.